Display a dual number
Invisible NULL; called for side effect of printing.
x <- dual(3, 1)
x # prints: <dual: 3 + 1*e>
#> <dual: 3 + 1*e>
dv <- dual_vector(dual(1, 0), dual(2, 1))
dv # prints: <dual_vector: 2 elements>
#> <dual_vector: 2 elements>
#> [1] <dual: 1 + 0*e>
#> [2] <dual: 2 + 1*e>